QJsonValue::toArray() wraps correct json array inside another array

Poggar

(QT 6.8.1 on linux with cmake)I found this weird behavior where the QJsonValue::toArray function returns the correct array, but wrapped inside another array and I'm not sure if I'm using the library correctly:
file.json:

{
  "success": true,
  "value_1": false,
  "array": [
    {
      "some_value_A": "AAA",
      "value_1" : "111"
    },
    {
      "some_value_B": "BBB",
      "value_2" : "222"
    },
    {
      "some_value_C": "CCC",
      "value_3" : "333"
    }
  ]
}

C++ Code:

const QJsonDocument json_doc{ json_document_from_file };
qDebug() << "json_doc.isObject() = " << json_doc.isObject(); // prints true
const QJsonObject object{ json_doc.object() };
qDebug() << "object.contains(\"array\") = " << object.contains("array"); //prints true
qDebug() << "object.size() = " << object.size(); //prints 3
const QJsonValue value{ object["array"] };
qDebug() << "value.isArray() = " << value.isArray(); //prints true
const QJsonArray array{ value.toArray() };
qDebug() << "array.size() = " << array.size() << "\t Should be 3"; //prints 1
qDebug() << "array.at(0).toArray().size() = " << array.at(0).toArray().size() << "\t Should be 3"; //prints 3
qDebug() << "value.toArray().size() = " << value.toArray().size() << "\t Should be 3"; //prints 3

hskoglund

Hi, it's easy to wrap JSON in too many layers, I think the culprit is this line
....
const QJsonArray array{ value.toArray() };
...
I think that QJSonArray array is a double wrapper, that's why you get size() = 1, because that's the size of the outer array (i.e. the outer array has 1 element, which is one array which has 3 elements :-).

SGaist

@Poggar hi,

@hskoglund is correct, you are not using a copy constructor but a constructor with an initializer list hence you are creating a new array out of the one you gave it since an array in an array is a valid use case.

The documentation could be clearer about that case though.

JonB

@Poggar
As the above two posts have said. Fastest way to convert Vector to QJsonArray? gives code and confirms there is no faster way than some sort of iteration over the C++ array adding elements to the JSON array, you can use std::copy() to save you writing a for loop yourself if you wish.